Donald Trump is smothering his opponents to a slow political death

ANALYSIS: When Donald Trump announced his run for the presidency in June he was viewed by the rest of the contenders as an unwelcome distraction who would drop out after a couple of weeks.

Instead he became an unwelcome frontrunner whose success has seen two perceived heavyweights in the Republican party quit their runs for lack of support.

On Tuesday, Wisconsin Governor Scott Walker gave up on his bid for the White House only a few months after he was viewed as one of the leading candidates.

Unsuccessful presidential candidate Scott Walker. (AAP) ()

His sudden departure came as a rude shock for his supporters, including an independent group of supporters who aired a new ad promoting him hours after his withdrawal from the race.

Usually the major candidates don't start dropping out until they've lost a few primaries. But the first ballot of the presidential race is still four months away, and Walker and former Texas governor Rick Perry have both already quit.

Normally a concession speech is a politician's best opportunity to be magnanimous, but Walker took the opportunity to launch a spiteful broadside at Trump, who has led nearly every poll for the past few months.

"Today, I believe that I am being called to lead by helping to clear the field in this race for a positive conservative message to rise to the top of the field," Walker said.

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"I encourage other Republican presidential candidates to consider doing the same so that the voters can focus on a limited number of candidates who can offer a positive conservative alternative to the current front-runner."

Walker's drop-out is a real concession that Trump is not the flash-in-the-pan success that the other candidates thought he would be. And though his bluster has been widely mocked and criticised in the media, Trump hasn't made the rest of the field look moderate or diplomatic or (dare I say it) presidential by comparison. Instead, he's made them look weak, ineffectual and boring.

What's worse for candidates like Jeb Bush, Marco Rubio and John Kasich, Trump has stolen all the oxygen from the race. When one candidate is making all the noise, it makes it hard for the rest of the field to be heard at all. And it's not just the media to blame. Trump has dominated press coverage of the race, but he's also winning the attention of ordinary voters. Thousands have been coming to his rallies while others have struggled to draw more than a few dozen at a time.

Worse still for 2012 star Rick Santorum, who nearly won Iowa in the last caucus. On June 8, the former senator held an event to which nobody at all showed up. He was filmed drinking a milkshake in the diner he was meant to be speaking at, alone.

In an extraordinary repudiation of the state of American politics, the second placed candidate in the race has never run for office before either. Ben Carson is an acclaimed neurosurgeon who has managed to claim the support of the substantial base of evangelical voters. Jeb Bush, who on paper should be the frontrunner, is barely breaking double digits in the polls.

For now the Republican field's strategy is to stay in the game long enough for Trump to screw up big. Then each contender hopes to be the one voters turn to. But presidential campaigns cost money, and it's not easy getting donations when America's most prominent billionaire is using a big microphone to call you a "loser". For all the candidates but one, that insult will be all too true.